1. Field of the Invention
The present invention relates to an audio amplifier, and more particularly, to a quaternary/ternary modulation selecting circuit of an audio amplifier and an associated method.
2. Description of the Prior Art
An analog amplifier is one of the most important parts in an audio system. Efficiency is a critical issue; a class D power amplifier is the most popular kind due to its higher efficiency compared to other kinds of amplifier. Because the output waveform of the class D power amplifier is a modulation signal between two voltage levels (i.e. supply voltage and ground) rather than a common linear waveform, ideally no current will pass through when the transistors of the output stage are non-conductive. A common modulation method applied in the class D power amplifier is Pulse Width Modulation (PWM), wherein quaternary modulation has the advantages of better total harmonic distortion (THD) and lower noise floor and is better for operating under small power conditions; and ternary modulation has the advantages of better efficiency and better Electro Magnetic Interference (EMI) performance and is better for operating under large power conditions. A quaternary/ternary modulation selecting circuit which can combine the above advantages is therefore a popular architecture. Pulses with extremely short duty cycle which cannot be detected by the circuit but still cause power loss will show at the output terminal when there is no input signal or a very small input signal for the amplifier. Therefore, it is desirable to develop a novel quaternary/ternary modulation selecting circuit design which can solve the power dissipation and distortion when there is no input signal for the amplifier.